Re: [RFC PATCH 3/3] kvm: use yield_to instead of sleep in kvm_vcpu_on_spin

Chris Wright
2010-12-03T02:25:13Z

* Rik van Riel (riel@redhat.com) wrote:
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1880,18 +1880,53 @@ void kvm_resched(struct kvm_vcpu *vcpu)
> }
> EXPORT_SYMBOL_GPL(kvm_resched);
>
> -void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu)
> +void kvm_vcpu_on_spin(struct kvm_vcpu *me)
> {
> - ktime_t expires;
> - DEFINE_WAIT(wait);
> + struct kvm *kvm = me->kvm;
> + struct kvm_vcpu *vcpu;
> + int last_boosted_vcpu = me->kvm->last_boosted_vcpu;

s/me->//

> + int first_round = 1;
> + int i;
>
> - prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> + me->spinning = 1;
> +
> + /*
> + * We boost the priority of a VCPU that is runnable but not
> + * currently running, because it got preempted by something
> + * else and called schedule in __vcpu_run. Hopefully that
> + * VCPU is holding the lock that we need and will release it.
> + * We approximate round-robin by starting at the last boosted VCPU.
> + */
> + again:
> + kvm_for_each_vcpu(i, vcpu, kvm) {
> + struct task_struct *task = vcpu->task;
> + if (first_round && i < last_boosted_vcpu) {
> + i = last_boosted_vcpu;
> + continue;
> + } else if (!first_round && i > last_boosted_vcpu)
> + break;
> + if (vcpu == me)
> + continue;
> + if (vcpu->spinning)
> + continue;
> + if (!task)
> + continue;
> + if (waitqueue_active(&vcpu->wq))
> + continue;
> + if (task->flags & PF_VCPU)
> + continue;

I wonder if you set vcpu->task in sched_out and then NULL it in sched_in
if you'd get what you want you could simplify the checks. Basically
that would be only the preempted runnable vcpus.

> + kvm->last_boosted_vcpu = i;
> + yield_to(task);

Just trying to think of ways to be sure this doesn't become just
yield() (although I thnk PF_VCPU is enough to catch that).